THIS AVALANCHE FORECAST EXPIRED ON March 26, 2018 @ 6:54 am
Avalanche Forecast published on March 25, 2018 @ 6:54 am
Issued by Brandon Schwartz - Tahoe National Forest

MODERATE avalanche danger continues near and above treeline due to an ongoing wind slab avalanche problem. Human triggered wind slab avalanches remain possible today. Below treeline, avalanche danger is LOW.

2. Moderate

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Above Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Wind Slab
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Moderate to strong SW winds combined with periods of snowfall have created several layers of wind slab near and above treeline on NW-N-NE-E-SE aspects. These wind slabs have been difficult to trigger but exist in widespread areas. Avalanche size up to D2 is possible (large enough to bury or injure a person).

With triggering difficult, determining areas of instability may be challenging. In order to identify areas of concern, look for recent wind slab avalanches, cornices above slopes, firm hollow sounding snow, blowing snow, and wind pillow type snow drifts. Use these clues to identify the areas to avoid, or use low angle slopes. More desirable conditions for recreation in the form of softer snow exist in wind sheltered terrain away from areas of wind slab.

Avalanche Problem 2: Storm Slab
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Storm slab avalanches are unlikely today, but have a level of concern for the isolated areas below treeline that received 10 to 14 inches of new snow over the past 24 hours. Periods of snowfall in the form of graupel (pellet snow) were observed yesterday and may provide a short lived weak layer within the top foot of the snowpack. Avalanche size is expected at D1 (not large enough to bury a person). The elevated consequences would come from being carried into or over down slope hazards such as cliffs, rocks, trees, or terrain traps.

Look for human triggered snow surface cracking in wind protected areas below treeline. Quick hand pits may be helpful in identifying a problematic weak layer within the top foot of the snowpack. Evaluate terrain carefully and avoid specific slopes with increased consequences in any areas where signs of potentially unstable storm slabs are found.

Forecast discussion

To the best of SAC's knowledge, triggering a deep slab avalanche has become very unlikely. For this reason it is no longer included as a specific avalanche problem. If you have unreported information relating to signs of deep slab instability occurring March 23 through the present, please submit an observation.

recent observations
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* Observations made and received yesterday in the Mount Rose and Donner summit areas noted widespread areas of blowing snow and wind slab formation near and above treeline. Wind slabs were difficult to trigger (stubborn). Some tests indicated that if slab failure could be triggered, propagation would occur.

* Wind slab avalanches were noted on March 23 in the Mount Rose area and West Shore Tahoe area, triggered by natural cornice collapses.

* Numerous storm slab crowns from March 22 have been observed on NE-E aspects below treeline on Incline Lake Peak (Mount Rose area). These avalanches failed on a density change within the storm snow. On March 24, data from snowpit tests in one of the crowns combined with several ski cuts on terrain of similar aspect and elevation indicated that this weak layer is no longer reactive.

* More crowns from deep slab avalanche activity occurring during the March 20 - 22 storm cycle have been discovered on N-NE-E aspects both above and below treeline in the 7,000' to 10,000' elevation range. No reports of post storm deep slab avalanches have been received.

Weather and CURRENT CONDITIONS
weather summary

The main period of accumulating snow showers is coming to an end this morning. Additional light snow showers may occur today, mainly this afternoon and evening. Decreasing cloud cover is forecast for today with air temperatures remained below average. Ridgetop winds remain out of the SW this morning at moderate to strong in speed. Winds are expected to decrease to light to moderate speed this afternoon. For tomorrow expect more sunshine, warmer air temperatures, and lighter winds from the NE.

CONDITIONS ALONG THE SIERRA CREST BETWEEN 8200 FT. AND 9200 FT. IN THE LAST 24 HOURS
Weather Station Map
Weather Station Table
6am temperature: 12 to 17 deg. F.
Max. temperature: 23 to 31 deg. F.
Average ridgetop wind direction: SW
Average ridgetop wind speed: 42 mph
Maximum ridgetop wind gust: 99 mph
New snowfall: Generally 5 to 7 in. Isolated areas up to 14 inches
Total snow depth: 86 to 122 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Sunday Sunday Night Monday
Weather: Mostly cloudy skies, becoming partly cloudy. Scattered snow showers. Mostly cloudy skies, becoming clear. Isolated snow showers in the evening. Sunny skies, becoming partly cloudy.
Temperatures: 26 to 31 deg. F. 10 to 15 deg. F. 30 to 35 deg. F.
Mid Slope Winds: SW to W Variable NE
Wind Speed: 10 to 20 mph with gusts to 30 mph in the morning, decreasing to around 10 mph. Light winds Up to 10 mph.
Expected snowfall: 0 to trace 0 to trace 0
For 8000 ft. to 9000 ft.
Sunday Sunday Night Monday
Weather: Mostly cloudy skies, becoming partly cloudy. Scattered snow showers. Mostly cloudy skies, becoming clear. Isolated snow showers in the evening. Sunny skies, becoming partly cloudy.
Temperatures: 21 to 27 deg. F. 8 to 13 deg. F. 24 to 30 deg. F.
Ridge Top Winds: SW NE NE
Wind Speed: 20 to 35 mph with gusts to 55 mph, decreasing to 10 to 15 mph with gusts to 30 mph in the afternoon. Light winds, becoming 10 to 15 mph with gust to 30 mph. 10 to 20 mph. Gusts up to 30 mph in the afternoon.
Expected snowfall: 0 to trace 0 to trace 0
Disclaimer

This avalanche advisory is provided through a partnership between the Tahoe National Forest and the Sierra Avalanche Center. This advisory covers the Central Sierra Nevada Mountains between Yuba Pass on the north and Ebbetts Pass on the south. Click here for a map of the forecast area. This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires 24 hours after the posted time unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.

For a recorded version of the Avalanche Advisory call (530) 587-3558 x258